In a lot of cases, I think the art is commissioned before the contents are finalized. Besides, Victorian Griffin still isn't in the game (it's a promo), although Crusader Griffin made an appearance in 2016's Paladin vs Siren.
Another obvious one is swarms of bees on the cover of Druid vs Necromancer.
Now if Mordok's Tome hadn't appeared in Lost Grimoire v1, I think people would have legitimately cried foul.